fscrypt: rename keyinfo.c to keysetup.c

Rename keyinfo.c to keysetup.c since this better describes what the file
does (sets up the key), and it matches the new file keysetup_v1.c.

Reviewed-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Eric Biggers <ebiggers@google.com>